The invention relates generally to optical-monitoring systems and more particularly to an optical arrangement for directing optical beams for monitoring and feedback.
Increasingly, data communications involve transmissions by optical sources that can deliver tremendous volumes of digitized information as pulses of light. This is especially true for many communication companies that utilize laser diodes and optical fibers as their primary means for the transmission of voice, television and data signals for ground-based communications networks.
To achieve high bandwidth, laser diodes such as Fabry-Perot lasers and Vertical Cavity Surface Emitting Lasers (VCSELs) are commonly utilized as optical sources. These types of laser diodes are preferred due to their minute dimensions. For example, the typical VCSEL is measured in the order of micrometers. Consequently, an array of laser diodes can be integrated into a system to achieve high bandwidth transmissions.
In many applications, the optimum output of a laser diode is dependent on the light intensity or the optical power output that is generated by the optical source. Many factors can affect the optical power output. For one, a temperature variation in the environment under which the diode is operating can affect the optical power output, even assuming that all the other variables remain constant. At a certain input current beyond the threshold level, the laser diode changes from the light emitting diode (LED) mode of operation into the laser mode. Under the LED mode, incoherent light is emitted that diffuses rapidly. This mode is inadequate for data transmissions over long distances due to a generally inherent significant loss of optical power. Under the laser mode, however, the light narrows into a coherent beam as the input current is increased. This mode is adequate for data transmissions over long distances. While the diodes of a particular class of diodes will have threshold levels in a relatively narrow range of input currents, the threshold level of a particular diode in the class can vary significantly with changes in the surrounding temperature. For example, the threshold level of some diodes can vary by as much as fifty percent as temperature changes cause the diodes to fluctuate between their normal threshold level and their deviated maximum level. Temperature variation can drastically affect the level of optical power output by a laser diode.
A second factor that can affect the optical power output is the age of the laser diode. As with other devices, the performance of the laser diode will degrade as a function of time. For one, the threshold level at which the laser diode switches from the LED mode to the laser mode can vary significantly due to the degradation of the diode from extended uses. Consequently, the integrity of data transmissions will be jeopardized.
As the demand for VCSELs increases, it is expected that no single manufacturer will have the capacity to provide all of the required laser diodes. With more manufacturers entering into the supply channel, VCSELs that fall within the same class having the same specifications will manifest dissimilar optical and electrical characteristics over prolonged uses. This is true even for VCSELs from the same batch produced by the same manufacturer. The problem is twofold. If the intensity of the beam is too high, the product may be harmful to the eyes. Eye safety is a factor that is considered in the design and testing of VCSELs. On the other hand, if the intensity is too low, the integrity of the transmitted data may be compromised. Thus, there is only a narrow band of optical power output level in which a VCSEL can operate to achieve satisfactory safety and performance.
Based on the various factors that can affect the performance of a VCSEL, the optical power output is regularly monitored and the input current to the emitting source is adjusted in order to ensure an optimum output for both operation and safety. Typically, a VCSEL is monitored by using a photo-detector, so that the optical power of the transmitted beam from the VCSEL is converted into electrical signal. The optical power output received by the photo-detector is used as a feedback signal that is coupled to a feedback controlling system, for adjusting and biasing the input current to the VCSEL.
In the case of a parallel channel optical array, an array of VCSELs having multiple channels is used for data communications. For a 1xc3x97n VCSEL array, there are n VCSELs with n corresponding channels for data transmissions. The n VCSELs are typically fabricated on a semiconductor substrate, such as a type III-V semiconductor wafer.
To ensure acceptable operational performance for the parallel channel optical array, the optical power outputs of the VCSELs are monitored and the input currents are responsively adjusted. One known arrangement for monitoring the optical power output of the VCSEL array is to monitor an end VCSEL of the array. As an example, in a 1xc3x97n VCSEL array, only the VCSEL that is located at the farthest end is monitored. Thus, if the 1xc3x97n array is numbered in sequence from the 1st VCSEL to the nth VCSEL, either the 1st VCSEL or the nth VCSEL is monitored. The VCSEL that is used for monitoring will not be used for data transmissions. This method presumes that the VCSEL that is monitored manifests optical and electrical characteristics that are representative of the other nxe2x88x921 VCSELs in the array. However, since only one VCSEL is monitored, this arrangement does not provide specific information on any particular VCSEL that is not monitored.
Another arrangement for providing monitoring of the optical power output of the VCSEL array is to monitor both ends of the array. In the same exemplary 1xc3x97n array, the 1st VCSEL and the nth VCSEL are both monitored. While a more accurate representation of the operational characteristics of the other VCSELs is provided, this arrangement suffers from two problems. First, no specific information is acquired on the other nxe2x88x922 VCSELs. Second, two fewer VCSELs in the array are used for data transmissions.
Consequently, what is needed is an arrangement that allows every laser diode in the parallel channel optical array to be monitored in order to ensure that every diode is operating at its target level.
The invention utilizes a diffractive optical arrangement (DOA) that is configured to diffract a portion of an input beam of every vertical cavity surface emitting laser (VCSEL) in a parallel channel optical array, so that all of the VCSELs can be monitored simultaneously. For every channel in the array, there is a detector for monitoring the optical power output of the associated VCSEL and a feedback system for adjusting the input current. The DOA comprises a collimating beam input region on one side, a beam output region on the opposite side, and a detection output region. The DOA is configured to pass a first portion of a beam from the collimating beam input region to the beam output region for data transmissions and to diffract a second portion from the collimating beam input region to the detection output region for monitoring. The collimating beam input region includes a diffractive feature, which may be that of a computer generated hologram (CGH) or may be a grating.
In one embodiment, the input beam that is emitted by a specific VCSEL impinges the DOA at the collimating beam input region having the diffractive feature. The second portion that is diffracted propagates through the DOA to a detection output region and onto a detector for monitoring the optical power output. In this embodiment, the VCSEL and detector are on opposite sides of the DOA.
In another embodiment, the second portion that is diffracted by the diffractive feature of the beam input region is reflected by a first reflective region at the opposite sides of the DOA. The reflected second portion is directed to the detection output region, which is on the same side of the DOA as the VCSEL.
In yet another embodiment, the DOA includes a sequence of optical regions which cooperate to reflect and direct the diffracted portion of the input beam to the detection region. The input beam that is emitted by any one of the VCSELs impinges the DOA at the associated collimating beam input region that includes the diffractive feature. The second portion that is diffracted propagates through the DOA to a first reflective region, where it is reflected and directed to a second reflective region. The second portion is reflected and directed by the second reflective region through the DOA to a third reflective region. The reflected portion from the third reflective region propagates through the DOA to the detection output region. The detection output region directs the second portion to a detector for beam monitoring and feedback adjustment.
In accordance with the invention, the detector can be made of silicon, gallium arsenide, indium gallium arsenide, or of a material that is sensitive to the wavelength of the input beam generated by the VCSEL. Moreover, the reflective regions may be mirrors, reflective coatings, or metallic coated surfaces (e.g., reflecting diffractive optical elements) for directing the second portion to a next region of interest. The next region of interest may be a subsequent reflective region or the detection output region.
In accordance with the preferred embodiment, there is an array of DOAs, with each DOA corresponding to a specific channel in the parallel channel optical array. In an alternative embodiment, there is a single DOA that corresponds to all the channels in the parallel array. That is, the single DOA is configured to diffract the second portions of all the input beams that are emitted from the VCSELs in the array.
While not critical, the collimating beam input region of the DOA may have a spiral-phase configuration. This design eliminates the feedback of the optical power back to the emitting source of the VCSEL. The detected optical power output will have a ring-shaped pattern.
The diffractive feature of the collimating beam input region may be that of a computer generated hologram (CGH) or a grating. The CGH may be one-dimensional or two-dimensional. In one embodiment, the one-dimensional CGH integration comprises eight discrete depths per period that is repeated along one axis. Due to the diffractive principles of the CGH, the detected optical power output is a series of similarly shaped patterns, e.g., ring-shaped patterns when the spiral phase is included. The ring-shaped pattern that is not deviated by the CGH is the 0th diffraction order, corresponding to the first portion for data transmissions. The ring-shaped pattern with the intensity at a large angle deviation (e.g., the 9th diffraction order) corresponds to the second portion for monitoring and feedback. Optical power outputs in the other diffraction orders that are not selected for data transmissions (i.e., 0th diffraction order) or for monitoring (i.e., 9th diffraction order) are considered as noise and cross talk signals and are kept to a minimum.
In another embodiment, the diffractive feature of the collimating beam input region is that of a two-dimensional CGH. The CGH integration comprises eight discrete depths per period that are repeated along two axes. The detected output shows a series of similarly shaped patterns similar to the one-dimensional CGH, except that the optical power output of the two-dimensional CGH can be, for example, two intersecting series of similarly shaped patterns. Alternatively, the diffractive feature of the collimating beam input region is that of a volume hologram. In yet another embodiment, the diffractive feature of that of a grating is used to diffract the second portion of the input beam for monitoring.
While the preferred embodiment is described as having the diffractive feature being integrated to the collimating beam input region for diffracting the second portion, the diffractive feature can be fabricated and configured into a separate element. In this alternative two-piece arrangement, the collimating beam input region is able to diffract the second portion of the input beam while being coupled to a separate diffractive element.
In every channel, the input current for the associated VCSEL in the array is adjusted by a closed-loop feedback system. A first source of a feedback signal is the optical power that is received by the detector. Additionally, a second source of the feedback signal can be the optimum optical power at the current temperature. The value of the optimum optical power is provided by a memory device having operating parameters corresponding to the surrounding temperatures that are measured by a temperature sensor. The feedback system utilizes both feedback signals for adjusting the input current to every VCSEL in the array.
One of the advantages of the invention is that by providing monitoring and feedback for each VCSEL of the parallel channel optical array the system ensures acceptable performance for every channel, guarding against unacceptable losses that might result from temperature variations and aging effects. Moreover, monitoring a channel does not limit that channel to being used to provide feedback signals. Rather, the monitored channel can provide signals for data transmissions, as well as for monitoring.